Forming an aluminum-containing alloy protective layer on metals



FORMING AN ALUMINUM-CONTAINING ALLOY PROTECTIVE LAYER ON METALS Joseph L. Greene, Berkley, and James C. Holzwarth,

Birmingham, Mich., assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware No Drawing. Filed Dec. 31, 1956, Ser. No. 631,466

10 Claims. (Cl. 1178) This invention pertains to aluminum coated articles and more particularly to a method of forming a protective layer; of thin, ductile aluminum-containing alloy at surfaces of such articles.

A flow diagram illustrating the process is as follows:

Nickel base alloy or cobalt base alloy Preheated and fluxed in salt bath at 1280 F. to 1400 F.

Dippediu moltenaluminum a 1250 F. to 1400 F.

Rinsed m fiuxing salt Immersed in aqueous solution of H01 and alkaline salt Heated for 1 hr. to 6 hrs. at 1700 F. to 2350 F.

It is desirable to aluminum coat certain metal articles in order to form a surface layer of an alloy of aluminum with the base metal of the article. Such an alloy layer is especially beneficial for aircraft engine turbine blades, which are exposed to elevated temperatures under highly oxidizing conditions. As is well known, high temperature alloy components, such as turbine buckets and nozzle guide vanes, of gas turbine engines are subject to extended periods of service at high temperatures under variable stress conditions. When such components are formed of certain high temperature nickel base alloys and cobalt base alloys, they possess excellent strength under most service conditions. However, the life of these turbine buckets and nozzle guide vanes is materially reduced because of inadequate resistance to oxide penetration. As a result, it has been found highly beneficial to provide these high temperature alloys with a protective layer of an alloy of the base metal with aluminum.

A successful method of aluminum coating nickel base alloys and cobalt base alloys is described and claimed in copending patent application S.N. 615,417, which was nited States Farm '1 Patented Jan. 3i, 196i filed on October 11, 1956, in the names of Dean K. Hanink and Erwin R. Price and is owned by the assignee of the present invention. This process provides a protective alloy layer which eliminates undesirable oxidation, particularly along grain boundaries or preferred crystallographic planes, of nickel base alloy and cobalt base alloy gas turbine buckets and nozzle guide vanes. As a result, such turbine blades will withstand higher operating temperatures that those previously used. The protective surface layer possesses sufficient ductility to yield with the base metal when the latter is expanding, contracting, or stretching due to creep elongation under stress at elevated temperature.

In the case of a nickel base alloy, although the alloy of aluminum with the base metal is referred to herein as an aluminum-nickel layer or an aluminum-nickel alloy layer, it will be understood that this layer consists essentially of aluminum in combination with all the various constituents in the nickel base alloy of which the blade is formed. Analogous terminology is similarly employed to describe the protective surface layer formed on cobalt base alloys. Examples of suitable nickel base alloys and cobalt base alloys are hereinafter set forth.

A thin stable layer of the aforementioned type of aluminum-nickel alloy or aluminum-cobalt alloy is formed at surfaces of nickel base alloy or cobalt base alloy turbine buckets and nozzle diaphragms in accordance with the invention described in the aforementioned patent application by initially providing these articles with a thin aluminum coating. This may be conveniently accomplished by an aluminum dipping operation. On difiusion heat treatment, the aluminum coating further combines with the base alloy to form a layer of aluminum-nickel alloy or aluminum-cobalt alloy. This layer will not flake or spall during the normal operating life of the engine in which turbine blades so treated are installed.

The diffused aluminum-nickel alloy and aluminumcobalt alloy layers are tough, resilient and possess good ductility. These alloy layers prevent oxide penetration of the base metal, thereby improving the durability of the turbine buckets under high temperature operating conditions. This protection is efiective under static stress conditions, non-load conditions, impact load conditions or hot working or forging conditions.

Of course, some of the aluminum usually remains on the surface of the aluminum-nickel or aluminum-cobalt layer in the form of a thin overlay of aluminum and aluminum oxides. An acid solution, preferably hydrochloric acid, is used to remove this overlay. Elimination of the aluminum overlay is essential in order to provide the outer surface of the coated article with proper ductility and to prevent spalling or chipping. However, in removing excess aluminum overlay from hot dipped aluminum coated turbine buckets formed of nickel base alloy or cobalt base alloy, the use of even dilute solutions of hydrochloric acid at room temperature frequently attacks the aluminum-nickel intermetallic compound layer. This process, which normally is carried out with about a 10% hydrochloric acid solution, is regarded as a critical operation since close control of time, temperature and concentration of the solution is neecssary to remove the aluminum overlay without damaging the aluminum-nickel alloy layer. Excessive chemical attack by the acid causes the intermetallic layer to lose its refractory and oxidation-resistant properties.

Normally the problem is not a serious one. However, if the aluminum overlay is not of generally uniform thickness, areas having a thin overlay will dissolve before the thicker areas, leaving the underlying aluminum-nickel compound layer exposed to the pickling solution. In some instances the acid solution normally used severely attacks these exposed areas and entirely removes the alloy layer from localized surfacesin' as short a period of time asv75 minutes.

A principal object of the present invention, therefore, is to provide a pickling solution which may be employed to remove the undesirable aluminum overlay without danger of'darnaging the protective aluminum-nickel alloyor aluminum-cobalt alloy layer by chemical'attack. This and otherobjects are attained in accordance with our invention by the use of anacid' pickling solution which efiectivelyremoves the aluminum overlaybut which is suitably-bufiered or inhibited so as to decrease its rate ofattack on the protective alloy layer. An alkaline salt, such as trisodium phosphate, has been found to be particularly effective in inhibiting a hydrochloric acid solution. We prefer to use trisodium-phosphate in an amount which reacts to'partially neutralize the activity of the hydrochloric acid and bufiers the hydrogen ion by the; introduction of an anion of a less highly ionized acid.

Accordingly, it appears that optimumresults are pro-- duced with an aqueous-pickling solution containing approximately 9% by volumeof concentrated hydrochloric acid and-85 grams per liter of trisodium phosphate. The concentration of the two active constituents inthe solution may vary somewhat, however. A hydrochloric'acid content between about 7% and 11% by volume issatisfactory, While the amount of trisodium phosphate may vary from approximately 70 grams per liter'to 100grams per liter. Concentrations of more than 11% hydrochloric acid and less than 70'grams'per'liter of trisodium phosphate lead to excessive attack of the aluminumnickel alloy layer. n the other hand, a solution containing less than 7% by volume of hydrochloric-acid and more than 100 grams per liter of trisodium phosphate dissolves the aluminum overlay so slowly as to be considered impractical for commercial processing.

The layer of aluminum-nickel alloy or aluminum-cobalt alloy, as the case may be, maybe formed at-thejsurfaces of the nickel base or cobalt 'baseturbine-bucket or nozzle diaphragm in any desired manner. The preferred'metlr od'is to apply molten aluminum or aluminum basealloy to the turbine blade under conditions such that the aluminum will form an alloy with the nickel or cobalt and result in the desired alloy layer thickness. Excellent results are obtained when the aluminum or aluminum base alloy is applied by the procedures described in United States Patent No. 2,569,097, Grange et al., owned by the assignee of the present invention.

An especially advantageous method comprises preheating the turbine blade to a temperature between approximately 1280 F. and 1400 F. in a fused salt bath consisting essentially of 37% to 57% KCl, 25% to 45% NaCl, 8% to 20% Na AlF and 0.5% to 12% AIF L The heated turbine blade is thereafter immersed for a short time in a molten bath of aluminum or aluminum base al- 10y at a temperature of about 1250 F; to 1400F. Ordinarily, the turbine blade being coated is retainedin the molten aluminum or aluminum base alloy not more than .approximately 10 seconds, a period between and seconds being preferred at present.

Subsequently, the turbine blade is removed from the aluminum bath and rinsed for a short period of time not in excess of approximately 15 seconds in the fluxing salt. Best results are obtained if the total dip time in the aluminum coating bath and the subsequent salt bath is be-- tween 10 and seconds. The excess coating material,

which is still in a semi-molten or mushy condition, is then:

removed by rapidly vibrating the turbine blade, preferably while it is still immersed in the salt. Alternatively, an air blast may be employed to remove the surplus coating material. As thus treated, the turbine blade is provided with an extremely thin and uniform coating of aluminum bonded to the nickel base metal or cobalt base metal by an intermediate extremely thin and uniform layer of an alloy of aluminum-nickel or aluminum-cobalt.

The surfaces of the turbine blade to be coated are pref aqueoussolution containing about 2% hydrofluoric acid,

7% sulfuric acid and 10% nitric acid. In. some. instances, the turbine blade mayrequire only a simple degreasing treatment in a chlorinated solvent prior to the aluminum coating and alloying operation. Mechanical cleaning methods, suchas grit blasting, sand blasting, hydroblasting, etc., may be employed in some cases to supplement the chemical treatment.

The steps of degreasing and pickling the turbine blade are. not essential to the process, however, as heatingin the fused salt prior to immersion inthe aluminum or aluminimum base alloy bath will provide the. turbine blade with clean surfaces unlessit had been exceptionally contaminated initially.

After the nickel base or cobalt base turbine blade has been cleaned, any portions thereof which are not to be coated, such as the attaching portions at the base of the blade, may be treated, with a suitablestop-ofi coating to prevent the aluminum from bonding to or alloying with the base metal at such surfaces. A suitable stop-off material for this purpose is a sodium silicate solution, such as an aqueous solution containing 20% to 50% sodium silicate.

It will be understood that variations in the aluminum coating method hereinbefore described may be made without departing from the scope of the present invention. For example, the aluminum may be applied to the turbine blade in the form of a paste or paint as described in co-pending patent application S.N. 459,093, Thomson et al., filed on September 29, 1954, now United States Patent No. 2,885,304, and owned by the assignee of the present invention. An example of the aluminum paste or paint which may be used is a mixture of aluminum powder with suitable amounts of a liquid vehicle, such as low low ash content lacquer or resin solution, liquid Lucite or a water solution of salt flux. The mixture of aluminum powder and resinous. carrier, such as vinyl or acrylic resins in appropriate organic solvents, may be applied by brushing, spraying or other appropriate means. A combination of resins or lacquers, salt fluxes and organic liquid vehicles also may be employed. Alternatively, the aluminum may be hot sprayed onto surfaces of the nickel base or cobalt base turbine blade, this method commonly being referred to as metallizing.

Whether the aluminum is applied in the form of a paste. or paint or as a hot spray, proper bonding, of the aluminum-coating. material to the nickel base alloy or cobalt base alloy may be effected by subsequent heating, such as lay-immersion in the aforementioned salt bath. If a-paste is used, it should be allowed to dry before immersing the turbine blade in the salt so as to avoid introducing volatile matter into the hot salt. This bath provides proper fluxing of the nickel base alloy orcobalt base alloy and simultaneously melts the coating metal or keeps it in'a. molten state so as to distribute the, aluminum thinly and evenly over the turbine blade. The molten salt thus prevents the formation of detrimental oxides which might otherwise adversely afilect the resultant bond at the interface of the aluminum-nickelalloy or aluminum-cobalt alloy and the base metal.

The aluminum or aluminum base alloy coating material should contain approximately or more aluminum in order to provide nickel base and cobalt base turbine blades with effective high-temperatureoxidation resistance. Hencethe terms aluminum and aluminum base alloy are interchangeably used herein as including not only pure aluminum and commercially pure aluminum, but also alloys containing at least approximately 80% aluminum. An alloy consisting essentially of about 2% iron and the balance aluminum provides excellent results. Another aluminum base alloy which is particularly advantageous to use is one composed of approximately 5% to silicon and the balance aluminum. This alloy has a relatively low melting temperature, i.e., eutectic at 12% silicon, and has high fluidity. Specific examples of other appropriate aluminum base alloys include an alloy composed of 4% copper and the balance aluminum, an alloy composed of approximately 7% tin or 7% silicon and 93% aluminum, and an alloy containing 5% to zinc and the balance substantially all aluminum. These specific examples are referred to merely for purposes of illustration and not of limitation.

Turbine rotor buckets, nozzle guide vanes and stator blades are all forms of turbine blades which are exposed to high operating temperatures in gas turbine engines, particularly of the axial flow type. All of these parts may be formed of high-temperature creep-resistant nickel base alloys and cobalt base alloys provided with a protective aluminum-nickel or aluminum-cobalt surface layer. Accordingly, the term turbine blades is employed herein as encompassing these various types of gas turbine engine components.

The aluminum-nickel alloy protective layer should in every instance be extremely thin. In general, the layer of this alloy should have a thickness of from approximately 0.0005 inch to 0.0025 inch. An aluminum-nickel layer 0.0010 inch to 0.0020 inch thick is preferred, however, with a layer thickness of about 0.0015 inch being considered optimum. Similar thicknesses are appropriate in the case of the aluminum-cobalt alloy protective layer. The thickness of the outer aluminum layer initially formed should not be in excess of approximately 0.004 inch, and it is presently preferred that this layer have a thickness less than about 0.0015 inch.

The following Table I contains examples of suitable high-temperature, creep-resistant nickel base alloys which may be satisfactorily provided with a thin, protective surface layer of aluminum-nickel in accordance with the present invention, the compositions being listed in terms of percent by Weight:

Table I Example Example Example Example Example 1 2 3 4 5 Carbon 0.15 Max. 0. 35-0. 45 0.07 0.15 0.08 Max. Max. Manganese-.- 1 Max. 2-3 1 Max. 1 Max 0. 3-1 Chromlum 15.5-17.5 23. 5-26. 5 19. 19 14-10 Cobalt 2 5 Max 10-15 13.5 10 Molybdenum 10-18 2-4 4. 10

. .5 T? 0.87 0.4-1 S1licon 1 Max 1 Max 0.75 0. 65 0.5

Max. Max. Max Sulfur 0.03 Max. 0.01 Mast.2 Co er pp Max. Nick Balance Balance Balance Balance Balance However, the nickel base alloy disclosed in United States Patent No. 2,688,536, Webbere et al., appears to be the most outstanding turbine bucket material currently available with respect to stress-rupture properties, creep resistance, ductility and high-temperature corrosion resistance when provided with a protective surface layer of aluminum-nickel. This alloy comprises approximately 0.06% to 0.25% carbon, 13% to 17% chromium, 4% to 6% molybdenum, 8% to 12% iron, 1.5% to 3% titanium, 1% to 4% aluminum, 0.01% to 0.5% boron and the balance substantially all nickel. For some applications the aluminum content may be increased to approximately 6 6% and the iron content may be as low as 0.1% or as high as 35%. The alloy usually should not contain more than 20% iron, however. Normally manganese and silicon not in excess of 1% each are also included in the alloy.

Examples of high-temperature cobalt base alloys which may be advantageously provided with a thin surface layer of aluminum-cobalt by the process described herein are listed in the following Table II, the composition again being given 111 percentages by Weight:

Table [1 Example 1 Example 2 Example 3 Example 4 Carbon 0. 120-0. 35 0. 45-0. 60 0.20 Max. 0.32-0.42 Manganese 1. 00-2. 00 0. 60-1. Silicon 0. 6') Max. 0. 30-0. Sulfur 0. 04 Max. 25. 00-30. 00 23. 00-28. 00 20. 00-22. 00 19. 00-2]. 00 k 1. 50-3. 50 9. 00-12. 00 18.00-22. 00 19. 00-21. 00 4. 50-6. 50 2. 50-3. 25 3. 50-4. 50 2. 00-3. 00 3. 50-5. 00 0. 75-1. 25 3. 00-4. 50 0. 08-0. 16 2. 00 Max. 2.00 Max. 5. 00 Max. Balance Balance 18. 00-22. 00 40. 00-44. 00

Diifusion heat treatment of the turbine blades after the aluminum coating operation may be beneficially employed to reduce the aluminum concentration in the surface alloy layer. Such a heat treatment is highly desirable to maintain high-temperature properties of the nickel base turbine blades, and it does not adversely affect the surface protection afiorded by the aluminum coating. This diffusion provides an alloy layer which is plastic at elevated temperatures during deformation with no loss of oxidation resistance. In general, a diffusion heat treatment at approximately 1700 F. to 2350 F. for one to six hours has proved to be advantageous, While a dilfusion period of three to six hours at a temperature between 1800 F. and 2100 F. is preferred at present. Highly satisfactory results have been obtained by a five hour diffusion heat treatment at 1800 F., followed by air cooling. A one to three hour heat treatment at a temperature of 2000 F. to 2150" F. is also very effective. It is desirable to subsequently vapor blast the turbine blades for inspection purposes.

Before the difiusion heat treatment the aluminumcoated nickel base alloy or cobalt base alloy turbine blades have an excess unalloyed aluminum layer over the aluminum-rich alloy layer. The thickness of the as-dipped aluminum-rich alloy layer may be controlled to some extent by both the temperature of the aluminum dip bath and the composition of the aluminum coating material. An alloy of 2% iron and 98% aluminum, for example, produces an aluminum-nickel layer which is thinner than an aluminum-nickel layer formed by coating with pure aluminum under the same conditions. The length of the total dip period also affects the thicknesses of the protective alloy surface layer. The term total dip period is used herein as meaning the total exposure time of the immersed article to molten aluminum and includes the periods of immersion in both the aluminum coating bath and the salt flux.

However, examination has indicated that the as-dipped aluminum-rich alloy layer does not solely control the final diffused layer thickness. The excess aluminum overlay, which forms more aluminum-rich alloy in the difiusion process, has a greater influence on the final layer thickness. Although vibration of the turbine blades in the salt bath removes some of the excess aluminum, an appreciable amount of the excess aluminum still remains on the surface of the blades.

Accordingly, to obtain optimum results by means of a thinner diffused layer of aluminum-nickel alloy or aluminum-cobalt alloy, it is advantageous to remove the excess aluminum overlay before diflusion. As indicated above, this can be effectively accomplished by immersing the as-dipped nickelbase or cobalt baseturbine blades,

in a dilute aqueous solution of hydrochloric acid which has been buffered by an alkaline salt such as, trisodium phosphate, Pickling for approximately one to two and one-half hours with a 9% hydrochloric acid solution at a temperature of about 60 F. to 90 F. has been found to produce excellent results. During this pickling or leaching operation it sometimes is advantageous to occasionally take the coated turbine blades out. of the leaching solution and scrub them with a fiber brush, for example, in order to remove loosely adhering insoluble particles from the surfaces of the blades. Under these circumstances it is desirable to rinse the blades in Water before re-immersing them in the leaching solution. We have also found it advisable to rinse the pickled blades in running water and scrub them before they are diffusion heatrtreatedr Of course, the pickling time employed for any particu: lar blade is governed to a large extent by the thickness of the aluminum overlay to be removed. However, even after the overlay has been dissolved and the aluminumrich alloy layer on a nickel base turbine bucket alloy has been exposed to this pickling solution for as long as 16 hours, this alloy layer is not destroyed. Hence it will be seen that this inhibited pickling solution provides great latitude and control of processing to insure a uniform, oxide-resistant coating on turbine blades.

Subsequent dilfusion produces an alloy layer less than 0.002 inch thick, as compared with a layer thickness of approximately 0.005 inch to 0.006 inch for aluminum coating nickel base specimens which are not pickled before difi'usion. Hence it can be seen that when the excess aluminum overlay is removed, the thickness of the final diffusedaluminum-nickel layer is controlled by the thickness of the aluminum-rich alloy which is formed during dipping. Since this thickness may be controlled by varying the total time the nickel base alloy is immersed in the aluminum and the salt bath, we have found a total dip period of to 20 seconds to be highly satisfactory. In this manner an aluminum-coated nickel base turbine blade may be produced which possesses excellent stressrupture and other physical properties, as well as good oxidation resistance at elevated temperatures.

Inyview of, the above facts, we have produced highly satisfactory results with a 10 second aluminum dip, a 10. second salt rinse, a 1 /2 hour pickling treatment in the above-described buffered hydrochloric acid solution, a five hour diffusion treatment at 1800 F. and air cooling. When a nickel base alloy, or cobalt base alloy is processed in this manner and subsequently exposed to two hundred hours of cyclic heating at a. temperature of approximately 1800" B, there is no evidence of oxide penetration into the base metal. The aluminum-nickel or aluminum-cobalt alloy layer on the surface of the base metal is relatively thin, an alloy layer having a thickness of only about 00017 inch to 0.0019 inch being typical. As a result, the aluminum-rich alloy layer is quite ductile and there is no indication that this layer tends to spall. The thin layer of aluminum-rich alloy possesses a low aluminum concentration at the surface. This combination of a thin aluminumuich layer and a low-aluminum concentration at its surface provides the treated alloy with optimum physical properties with respect to stressrupture characteristics and high-temperature oxidation resistance. Moreover, .the above-described process does not measurably increase the size of the treated article.

While this invention has been described by means of certain specific examples, it will be understood that the scope of the invention is not to be limited thereby except as defined in the following claims.

We claim:

1. A method of treating a turbine blade formed of a base metal selected from the class consisting of nickel base alloys and cobalt base alloys to increase its oxidation resistance at elevated temperature, said method comprising applying a coating metal selected from the class consisting of aluminum and aluminumbase alloys to surfaces of said turbineblade, heating said blade to diffuse a portion of said coating metal into said base metal, treating the solidified coating on said surfaces with an aqueous acid pickling solution containing hydrochloric acid and an alkaline salt for a period of time sufficient to remove from said surfaces substantially all of said coating which has not diifused into said base metal, and thereafter heating said treated blade for a period of time sufficient to further diffuse said coating metal into said surfaces to form with the base metal of said blade an oxidation-resistant surface layer of an alloy of said coating metaland said base metal.

2. A method of treating a turbine blade to increase its oxidation resistance at elevated temperature, said method comprising applying a coating metal selected from the class consisting of aluminum and aluminum base alloys to surfaces of a turbine blade formed of a base metal selected from the class consisting of nickel base alloys and cobalt base alloys, heating said blade to diffuse a portionrof said coating metal into said base metal, immersing said coated blade in an aqueous acid pickling solution containing hydrochloric acid and trisodium phosphate for one to two and one-half hours to remove from said blade most of the solidified coating which has not diifused into said base metal, and thereafter heating said treated turbine blade for a period of time sufiicient to further diffuse said coating metal into said surfaces to form with the base metal of said blade an oxidationresistant surface layer of an alloy of said coating metal and said base metal.

3. A method of treating an article formed of a base metal selected from the class consisting of nickel base alloys and cobalt base alloys in order to increase the high-temperature oxidation resistance of said article, said method comprising coating surfaces of said article with a thin layer of a coating metal containing at least aluminum, heating said article to diffuse a portion of said coating metal into said base metal, immersing said coated article in an aqueous pickling solution containing 7% to 11% by volume of hydrochloric acid and 70 to grams per liter of trisodium phosphate for one to two and one-half hours to remove from said article substantially all of the solidified coating which has not diffused into said base metal, and thereafter heating said treated article for a period of time sufiicient to further diffuse said coating metal into said base metal to form therewith an oxidation-resistant surface layer of an alloy of said coating metal and said base metal.

4. A method of providing an oxidation-resistant alloy layer at surfaces of an article formed of a base metal selected from the class consisting of nickel base alloys and cobalt base alloys, said method comprising immersing said article for a short period of time in a fused salt capable of absorbing aluminum oxides and applying a thin coating of a metal containing at least 80% aluminum to said surfaces, said salt being at a temperature of approximately 1280 F. to 1400 F. while said article is immersed therein, heating said article to diffuse a portion of said coating metal into said base metal, thereafter pickling said coated article in an aqueous solution containing minor amounts of hydrochloric acid and trisodium phosphate for a period of time sufficient to remove substantially all of the solidified coating on said article which has not diffused into said base metal, and subsequently heating said treated article to further dilfuse said coating metal into said base metal.

5. A'method of providing a turbine blade formed of a nickel base alloy with a thin oxidation-resistant surface layer of aluminum-nickel alloy, said method comprising immersing said blade in a molten salt capable of dissolving aluminum oxides and applying a layer of aluminum to the blade, said salt being maintained at a temperature between approximately 1280 F. and 1400" F. while a d de s mm rsed h n to dif u r u ng of said aluminum into said nickel base alloy, removing said blade from said salt and permitting the molten aluminum on said blade to solidify, immersing said coated blade for one to two and one-half hours in an aqueous acid pickling solution containing 7% to 11% hydrochloric acid and a minor proportion of an alkaline salt buffer to remove substantially all of the solidified aluminum on said blade which has not diffused into said nickel base alloy, and thereafter heating said treated blade at a temperature of about 1700 F. to 2350 F. for one to six hours to further diffuse aluminum into said nickel base alloy and form an aluminum-nickel surface layer having a thickness of approximately 0.0005 inch to 0.0025 inch.

6. A method of forming a thin oxidation-resistant layer of aluminum-nickel alloy on surfaces of an article formed of a nickel base alloy, said method comprising immersing said article in a fused salt comprising, by weight, approximately 37% to 57% KCl, 25% to 45% NaCl, 8% to 20% Na AlF and 0.5% to 12% AlF said fused salt being activated by aluminum in contact therewith, subsequently immersing said article in a molten bath of a coating metal containing at least 80% aluminum to form on said nickel base alloy a thin coating metal overlay bonded to said nickel base alloy by a thin diffusion layer of an alloy of said coating metal and said nickel base alloy, removing the coated article from said coating metal bath and permitting said overlay to solidify, subsequently pickling said coated article in a solution comprising 7% to 11% by weight of hydrochloric acid, 70 to 100 grams per liter of trisodium phosphate and the balance substantially all water for a period of time sufficient to dissolve substantially all of said overlay from said article, and thereafter heating said treated article for a short period of time to cause further diffusion of the coating metal into said nickel base alloy.

7. A method of providing a nickel base turbine blade with a thin protective surface layer of aluminum-nickel alloy, said method comprising applying a coating metal containing at least 80% aluminum to surfaces of said blade, immersing said blade in a molten salt comprising by Weight approximately 37% to 57% KC], 25% to 45% NaCl, 8% to 20% Na AlF and 0.5% to 12% AlF said molten salt being activated by aluminum in contact therewith, the portion of said blade being coated having a temperature at least as high as the melting point of said coating metal while in said activated salt to diffuse a portion of said coating metal on said surfaces into said nickel base alloy, removing the coated blade from said salt and permitting the molten coating metal on said blade to solidify, thereafter treating the coated surfaces of said blade with a dilute aqueous solution of hydrochloric acid and trisodium phosphate to remove substantially all of the coating metal on said surfaces which has not diffused into said nickel base alloy, and thereafter heating said treated turbine blade at a temperature of about 1700 F. to 2350 F. to further diffuse said coating metal into said nickel base alloy.

8. A method of forming a high-temperature oxidationresistant turbine blade which comprises cleaning a turbine blade formed of a base metal selected from the class consisting of nickel base alloys and cobalt base alloys, preheating and fluxing said turbine blade in a molten salt bath at a temperature of about 1280 F. to 1400 F., thereafter dipping said turbine blade in a molten aluminum bath for a period of time not in exess of approximately 15 seconds to coat surfaces of said turbine blade with aluminum, rinsing said coated blade in a molten salt bath to drain excess aluminum therefrom, removing said coated blade from said salt bath, permitting said aluminum to solidify to thereby form on said base metal a thin aluminum overlay bonded to said base metal by a thin intermediate diffused layer of an alloy of aluminum and the base metal, thereafter immersing said treated blade in an aqueous solution containing 7% to 11% by volume of hydrochloric acid and to 100 grams per liter of trisodium phosphate to remove substantially all of said overlay from said blade, and thereafter heating said treated blade to further diffuse the aluminum in said diffused layer into said base metal.

9. A method of forming a high-temperature oxidationresistant turbine blade having a thin layer of aluminumnickel alloy at its surfaces, said method comprising casting into the shape of a turbine blade a nickel base alloy comprising 0.06% to 0.25% carbon, 13% to 17% chromium, 4% to 6% molybdenum, 1% to 6% aluminum, 1.5% to 3% titanium, iron not in exess of 20%, boron not in excess of 0.5% and the balance substantially all nickel, thereafter coating said surfaces with a thin layer of aluminum and heating said coated blade to diffuse a portion of the aluminum coating into said nickel base alloy, thereafter treating the coated surfaces of said blade with an aqueous solution containing 7% to 11% by volume of hydrochloric acid and 70 grams to 100 grams per liter of trisodium phosphate to remove from said blade substantially all of said aluminum coating which has not diffused into said nickel base alloy, and finally heating said treated blade to cause further diffusion of said aluminum into said nickel base alloy.

10. A method of forming a high-temperature oxidation-resistant turbine blade which comprises preheating and fluxing surfaces of a nickel base alloy turbine blade in a molten salt bath capable of absorbing aluminum oxides, said salt bath being at a temperature of 1280 F. to 1400 F., immersing said fluxed turbine blade in a molten aluminum bath at a temperature of 1250 F. to 1400 F. for a short period of time to coat said fluxed surfaces with aluminum, removing said coated blade from said aluminum bath, permitting the aluminum on said surfaces to solidify to thereby form on said nickel base alloy a thin aluminum overlay bonded to said alloy with a thin intermediate layer of aluminum-nickel alloy, thereafter pickling the coated surfaces of said blade for approximately one to two and one-half hours in a dilute aqueous acid solution of hydrochloric acid and trisodium phosphate at a temperature of about 60 F. to F. to remove substantially all of said overlay from said blade, and subesquently heating said treated blade for one to six hours in a gaseous atmosphere at a temperature between 1700 F. and 2350" F. to further diffuse the aluminum in said aluminum-cobalt alloy layer into the base metal of said blade.

References Cited in the file of this patent UNITED STATES PATENTS 1,817,174 Brock Aug. 4, 1931 2,167,701 Whitefield et al. Aug. 1, 1939 2,569,097 Grange et al. Sept. 25, 951 2,662,270 Mitchell et al Dec. 15, 1953 2,688,536 Callaway et al. Sept. 7, 1954 2,738,289 Hodge Mar. 13, 1956 2,755,542 Boegehold July 24, 1956 2,774,686 Hodge et al Dec. 18, 1956 

1. A METHOD OF TREATING A TURBINE BLADE FORMED OF A BASE METAL SELECTED FROM THE CLASS CONSISTING OF NICKEL BASE ALLOYS AND COBALT BASE ALLOYS TO INCREASE ITS OXIDATION RESISTANCE AT ELEVATED TEMPERATURE, SAID METHOD COMPRISING APPLYING A COATING METAL SLECTED FROM THE CLASS CONSISTING OF ALUMINUM AND ALUMINUM BASE ALLOYS TO SURFACES OF SAID TURBINE BLADE, HEATING SAID BLADE TO DIFFUSE A PORTION OF SAID COATING METAL INTO SAID BASE METAL, TREATING THE SOLIDIFIED COATING ON SAID SURFACES WITH AN AQUEOUS ACID PICKLING SOLUTION CONTAINING HYDROCHLORIC ACID AND AN ALKALINE SALT FOR A PERIOD OF TIME SUFFICIENT TO REMOVE FROM SAID SURFACES SUBSTANTIALLY ALL OF SAID COATING WHICH HAS NOT DIFFUSED INTO SAID BASE METAL, AND THEREAFTER HEATING SAID TREATED BLADE FOR A PERIOD OF TIME SUFFICIENT TO FURTHER DIFFUSE SAID COATING METAL INTO SAID SURFACES TO FORM WITH THE VASE METAL OF SAID BLADE AN OXIDATION-RESISTANT SURFACE LAYER OF AN ALLOY OF SAID COATING METAL AND SAID BASE METAL.
 5. A METHOD OF PROVIDING A TURBINE BLADE FORMED OF A NICKEL BASE ALLOY WITH A THIN OXIDATION-RESISTANT SURFACE LAYER OF ALUMINUM-NICKEL ALLOY, SAID METHOD COMPRISING IMMERSING SAID BLADE IN A MOLTEN SALT CAPABLE OF DISSOLVING ALUMINUM OXIDES AND APPLYING A LAYER OF ALUMINUM TO THE BLADE, SAID SALT BEING MAINTAINED AT A TERMPERATURE BETWEEN APPROXIMATELY 128*F. AND 1400*F. WHILE SAID BLADE IS IMMERSED THEREIN TO DIFFUSE A PORTION OF SAID ALUMINUM INTO SAID NICKEL BASE ALLOY, REMOVING SAID BLADE FORM SAID ALT AN PERMITTING THE MOLTEN ALUMINUM ON SAID BLADE TO SOLIDIFY, IMMERSING SAID COATED BLADE FOR ONE TO TWO AND ONE-HALF HOURS IN AN AQUEOUS ACID PICKLING SOLUTION CONTAINING 7% TO 11% HYDROCHLORIC ACID AND A MINOR PROPORTION OF AN ALKALINE SALT BUFFER TO REMOVE SUBSTANTIALLY ALL OF THE SOLIDIFIED ALUMINUM ON SAID BLADE WHICH HAS NOT DIFFUSED INTO SAID NICKEL BASE ALLOY, AND THEREAFTER HEATING SAID TREATED BLADE AT A TEMPERATURE OF ABOUT 1700*F. TO 2350*F. FOR ONE TO SIX HOURS TO FURTHER DIFFUSE ALUMINUM INTO SAID NICKEL BASE ALLOY AND FORM AN ALUMINUM-NICKEL SURFACE LAYER HAVING A THICKNESS OF APPROXIMATELY 0.00005 INCH TO 0.0025 INCH. 